1. Field of the Invention
The present invention relates to an amplification device of a small detection signal with a sensor element, which amplifies an output signal of the sensor element by a operational amplifier. More specifically, the present invention relates to an amplification device for a small detection signal of sensor elements, such as a plurality of thermopile type infrared sensor elements, outputting a small detection signal of DC and low frequency range, which requires a means to prevent noise in the low frequency range.
2. Related Background Art
The recent high integration and low power consumption CMOS technology contributes largely to the miniaturization and power consumption reduction of the sensor devices. However, in the amplifier configured with differential amplification circuit such as CMOS operational amplifier, the input offset voltage caused by the threshold voltage variation is amplified to create an output offset voltage at an output terminal. The output offset voltage (referred to as “offset voltage” hereafter) becomes large enough, for example when CMOS operational amplifier is used in the amplification circuit in the infrared sensor that outputs a small detection signal of DC and low frequency, to push the amplifier out of linear operational range and may even create clipping state. Also especially when the small detection signal of DC and low frequency is input, the countermeasure against the noise in the low frequency range becomes a further problem in addition to the above offset voltage.
Up to now, the compensation method is known wherein a plurality of infrared sensor elements and the compensation elements having the same electrical characteristics but its signal value do not change even with infrared input are arranged contiguously. The offset value of the amplification circuit is obtained by operating a compensation element with a scanning means so that the output of sensor element is compensated, based upon the above obtained offset value as disclosed in Japanese Unexamined Patent Application Publication No. I19-218090.
However, the above described compensation method relates to offset voltage and the amplification factor of the amplification circuit is constant. Therefore, if the output of the sensor element is not uniform, there is an inconvenience that the output signal after amplification may be too small or too large. The object of the present invention is to avoid the above inconvenience and to provide an amplification device for the small detection signal of a multi-element sensor wherein an offset voltage of the multi-element sensor can be cancelled and an amplification factor of an operational amplification in the amplification circuit can be adjusted according to the output signal level of each sensor element.
The reference H06-45875 (Japanese Unexamined Patent Application Publication) discloses a switched capacitor amplifier circuit where a capacitor is formed between an inverting input terminal of an operational amplifier and reference voltage. For the cancellation actuation of an offset voltage, the capacitor becomes the load of the operational amplifier only at the time of cancellation actuation of the offset voltage. However, in the amplifier circuit disclosed by this reference, since variation in each capacitor resulting from the manufacturing process arises, it is difficult to fully cancel the voltage of offset. Furthermore, the noise in a low frequency cannot be reduced.
The reference H06-54118 (Japanese Unexamined Patent Application Publication) discloses a driving device of an image sensor having the following characteristic feature. FIG. 7 shows the driving device of the image sensor shown in the reference. A signal line L(1) receives an information signal detected by the photo detector of image sensors, and positive phase input terminal of an operational amplifier 21 is connected to this signal line. An output terminal of the operational amplifier 21 is connected to a positive input terminal of an operational amplifier 22 through an analog switch 24 and a capacitor 25. An analog switch 26 and a capacitor 27 are connected in juxtaposition between the positive input terminal (point A) and a ground potential. An output terminal of an operational amplifier 22 is connected to a positive input terminal of an operational amplifier 23 through an analog switch 28 and a capacitor 29. An analog switch 30 and a capacitor 31 are connected in juxtaposition between this positive input terminal (point B) and the ground potential. A switch 4 (1) is connected between the positive input terminal of the operational amplifier 21 and the ground potential. An analog switch 5 (1) is connected between the output terminal of the operational amplifier 23, and an output line OUTL. The operation of this circuit is explained as follows. At the time t1, capacitors 25 and 29 will be charged by setting analog switches 24, 26, 28 and 30 to ON. The electrical potential difference VC25 between terminals of capacitor 25 serves as VC25=20 (Vin+Vos1). The electrical potential difference VC29 between terminals of capacitor 29 serves as VC29=20 Vos2. (in this equality, Vin is input voltage, Vos1 and Vos2 are the input offset voltage of the operational amplifier 21 and 22, respectively). Next, at the time t2, the potential accumulated in signal-line L (1) will be discharged by setting analog switches 24, 26, 28 and 30 to OFF and the switch 4 (1) to ON. Then, at the time t3, by setting analog switches 24 and 28 to ON, electrical potential VA of point A will serves as VA=(20 Vos1−Vc25)/2+Vos2−−Vin+Vos3, and electrical potential VB of point B will serves as VB=(20(−10 Vin+Vos2)−Vc29)/2+Vos3=−100 Vin+Vos3. (in this equality, Vos3 is input offset voltage of the operational amplifier 23). Thereby, the input offset voltage Vos1 and Vos2 of operational amplifiers 21 and 22, respectively, are canceled, and then the whole circuit gain can be made into 100 times.
Concerning to the circuit disclosed in this reference, the positive terminals of the operational amplifiers 22 and 23 will be in a floating state from time t2 to time t3. Therefore, the electric charge charged by the capacitor 25 and 29 must continue being held during the period (time t2 to t3). The capacitors 27 and 29 formed for holding of the electric charge, and capacity with the big capacitor 25, 27, 29 and 31 is needed. Moreover, since it is dependent on the accuracy of capacitors 27 and 31, the accuracy of offset voltage is difficult to control with high precision. Furthermore, since the analog switches 24 and 28 are formed on the signal line of the operational amplifier, the noises by switching operation and the transmission delay by parasitism capacity arise.